Add socket option to query the current MPS

This enables applications to query the current value of the
Maximum Packet
Size via a socket option, suggested as a SHOULD in (RFC
4340, p. 102).

Signed-off-by: Gerrit Renker <gerriterg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acmeghostprotocols.net>
---
 Documentation/networking/dccp.txt |    3 +++
 include/linux/dccp.h              |    1 +
 net/dccp/proto.c                  |    4 ++++
 3 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/Documentation/networking/dccp.txt
b/Documentation/networking/dccp.txt
index 4504cc5..521e2dd 100644
--- a/Documentation/networking/dccp.txt
+++ b/Documentation/networking/dccp.txt
 -41,6
+41,9  the socket will fall back to 0 (which means that no
meaningful service code
 is present). Connecting sockets set at most one service
option; for
 listening sockets, multiple service codes can be
specified.
 
+DCCP_SOCKOPT_GET_CUR_MPS is read-only and retrieves the
current maximum packet
+size (application payload size) in bytes, see RFC 4340,
section 14.
+
 DCCP_SOCKOPT_SEND_CSCOV and DCCP_SOCKOPT_RECV_CSCOV are
used for setting the
 partial checksum coverage (RFC 4340, sec. 9.2). The default
is that checksums
 always cover the entire packet and that only fully covered
application data is
diff --git a/include/linux/dccp.h b/include/linux/dccp.h
index c297212..dcaa1ac 100644
--- a/include/linux/dccp.h
+++ b/include/linux/dccp.h
 -204,6
+204,7  struct dccp_so_feat {
 #define DCCP_SOCKOPT_SERVICE		2
 #define DCCP_SOCKOPT_CHANGE_L		3
 #define DCCP_SOCKOPT_CHANGE_R		4
+#define DCCP_SOCKOPT_GET_CUR_MPS	5
 #define DCCP_SOCKOPT_SEND_CSCOV		10
 #define DCCP_SOCKOPT_RECV_CSCOV		11
 #define DCCP_SOCKOPT_CCID_RX_INFO	128
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 7337e39..e00359d 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
 -586,6
+586,10  static int do_dccp_getsockopt(struct sock *sk, int
level, int optname,
 	case DCCP_SOCKOPT_SERVICE:
 		return dccp_getsockopt_service(sk, len,
 					       (__be32 __user *)optval, optlen);
+	case DCCP_SOCKOPT_GET_CUR_MPS:
+		val = dp->dccps_mss_cache;
+		len = sizeof(val);
+		break;
 	case DCCP_SOCKOPT_SEND_CSCOV:
 		val = dp->dccps_pcslen;
 		len = sizeof(val);
-- 
1.5.0.6

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